3GPP LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme. In radio communication systems to which 3GPP LTE is applied, base stations transmit synchronization signals (i.e., Synchronization Channel: SCH) and broadcast signals (i.e., Broadcast Channel: BCH) using predetermined communication resources. Meanwhile, each terminal finds an SCH first and thereby ensures synchronization with a base station. Subsequently, the terminal reads BCH information to acquire base station-specific parameters such as a frequency bandwidth (see, Non-Patent Literature (hereinafter, abbreviated as NPL) 1, 2 and 3).
In addition, upon completion of the acquisition of the base station-specific parameters, each terminal performs a connection request to the base station to thereby establish a communication link with the base station. The base station transmits control information via a downlink control channel such as Physical Downlink Control CHannel (PDCCH) as appropriate to the terminal with which a communication link has been established.
The terminal performs “blind-determination” on each of a plurality of control information items included in the received PDCCH (i.e., Downlink (DL) Assignment Control Information: also referred to as Downlink Control Information (DCI)). Specifically, each of the control information items includes a Cyclic Redundancy Check (CRC) part and the base station masks this CRC part using the terminal ID of the transmission target terminal. Accordingly, until the terminal demasks the CRC part of the received control information item with its own terminal ID, the terminal cannot determine whether or not the control information item is intended for the terminal. In this blind-determination, if the result of demasking the CRC part indicates that the CRC operation is OK, the control information item is determined as being intended for the terminal.
Moreover, in 3GPP LTE, Automatic Repeat Request (ARQ) is applied to downlink data to terminals from a base station. Specifically, each terminal feeds back response signals indicating the result of error detection on the downlink data to the base station. Each terminal performs a CRC on the downlink data and feeds back Acknowledgment (ACK) when CRC=OK (no error) or Negative Acknowledgment (NACK) when CRC=Not OK (error) to the base station as response signals. An uplink control channel such as Physical Uplink Control Channel (PUCCH) is used to feed back the response signals (i.e., ACK/NACK signals (hereinafter, may be referred to as “A/N,” simply)).
The control information to be transmitted from a base station herein includes resource assignment information including information on resources assigned to the terminal by the base station. As described above, PDCCH is used to transmit this control information. The PDCCH includes one or more L1/L2 control channels (L1/L2 CCH). Each L1/L2 CCH consists of one or more Control Channel Elements (CCE). More specifically, a CCE is the basic unit used to map the control information to PDCCH. Moreover, when a single L1/L2 CCH consists of a plurality of CCEs (2, 4 or 8), a plurality of contiguous CCEs starting from a CCE having an even index are assigned to the L1/L2 CCH. The base station assigns the L1/L2 CCH to the resource assignment target terminal in accordance with the number of CCEs required for reporting the control information to the resource assignment target terminal. The base station maps the control information to physical resources corresponding to the CCEs of the L1/L2 CCH and transmits the mapped control information.
In addition, CCEs are associated with component resources of PUCCH (hereinafter, may be referred to as “PUCCH resource”) in a one-to-one correspondence. Accordingly, a terminal that has received an L1/L2 CCH identifies the component resources of PUCCH that correspond to the CCEs forming the L1/L2 CCH and transmits response signals to the base station using the identified resources. However, when the L1/L2 CCH occupies a plurality of contiguous CCEs, the terminal transmits the response signals to the base station using a PUCCH component resource corresponding to a CCE having a smallest index among the plurality of PUCCH component resources respectively corresponding to the plurality of CCEs (i.e., PUCCH component resource associated with a CCE having an even numbered CCE index). In this manner, the downlink communication resources are efficiently used.
As illustrated in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread using a Zero Auto-correlation (ZAC) sequence having the characteristic of zero autocorrelation in time-domain, a Walsh sequence and a discrete Fourier transform (DFT) sequence, and are code-multiplexed in a PUCCH. In FIG. 1, (W0, W1, W2, W3) represent a length-4 Walsh sequence and (F0, F1, F2) represent a length-3 DFT sequence. As illustrated in FIG. 1, ACK or NACK response signals are primary-spread over frequency components corresponding to 1 SC-FDMA symbol by a ZAC sequence (length-12) in frequency-domain. More specifically, the length-12 ZAC sequence is multiplied by a response signal component represented by a complex number. Subsequently, the ZAC sequence serving as the response signals and reference signals after the primary-spread is secondary-spread in association with each of a Walsh sequence (length-4: W0-W3 (may be referred to as Walsh Code Sequence)) and a DFT sequence (length-3: F0-F2). To put it more specifically, each component of the signals of length-12 (i.e., response signals after primary-spread or ZAC sequence serving as reference signals (i.e., Reference Signal Sequence)) is multiplied by each component of an orthogonal code sequence (i.e., orthogonal sequence: Walsh sequence or DFT sequence). Moreover, the secondary-spread signals are transformed into signals of length-12 in the time-domain by inverse fast Fourier transform (IFFT). A CP is added to each signal obtained by IFFT processing, and the signals of one slot consisting of seven SC-FDMA symbols are thus formed.
The response signals from different terminals are spread using ZAC sequences each corresponding to a different cyclic shift value (i.e., index) or orthogonal code sequences each corresponding to a different sequence number (i.e., orthogonal cover index (OC index)). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. In addition, an orthogonal code sequence is referred to as a block-wise spreading code in some cases. Thus, base stations can demultiplex the code-multiplexed plurality of response signals using the related art despreading and correlation processing (see, NPL 4).
However, it is not necessarily true that each terminal succeeds in receiving downlink assignment control signals because the terminal performs blind-determination in each subframe to find downlink assignment control signals intended for the terminal. When the terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal would not even know whether or not there is downlink data intended for the terminal on the downlink component carrier. Accordingly, when a terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal generates no response signals for the downlink data on the downlink component carrier. This error case is defined as discontinuous transmission of ACK/NACK signals (DTX of response signals) in the sense that the terminal transmits no response signals.
In 3GPP LTE systems (may be referred to as “LTE system,” hereinafter), base stations assign resources to uplink data and downlink data, independently. For this reason, in the 3GPP LTE system, terminals (i.e., terminals compliant with LTE system (hereinafter, referred to as “LTE terminal”)) encounter a situation where the terminals need to transmit uplink data and response signals for downlink data simultaneously in the uplink. In this situation, the response signals and uplink data from the terminals are transmitted using time-division multiplexing (TDM). As described above, the single carrier properties of transmission waveforms of the terminals are maintained by the simultaneous transmission of response signals and uplink data using TDM.
In addition, as illustrated in FIG. 2, the response signals (i.e., “A/N”) transmitted from each terminal partially occupy the resources assigned to uplink data (i.e., Physical Uplink Shared CHannel (PUSCH) resources) (i.e., response signals occupy some SC-FDMA symbols adjacent to SC-FDMA symbols to which reference signals (RS) are mapped) and are thereby transmitted to a base station in time-division multiplexing (TDM). In FIG. 2, “subcarriers” in the vertical axis of the drawing are also termed as “virtual subcarriers” or “time contiguous signals,” and “time contiguous signals” that are collectively inputted to a discrete Fourier transform (DFT) circuit in a SC-FDMA transmitter are represented as “subcarriers” for convenience. More specifically, optional data of the uplink data is punctured due to the response signals in the PUSCH resources. Accordingly, the quality of uplink data (e.g., coding gain) is significantly reduced due to the punctured bits of the coded uplink data. For this reason, base stations instruct the terminals to use a very low coding rate and/or to use very large transmission power so as to compensate for the reduced quality of the uplink data due to the puncturing.
Meanwhile, the standardization of 3GPP LTE-Advanced for realizing faster communications than 3GPP LTE is in progress. 3GPP LTE-Advanced systems (may be referred to as “LTE-A system,” hereinafter) follow LTE systems. 3GPP LTE-Advanced introduces base stations and terminals capable of communicating with each other using a wideband frequency of 40 MHz or greater to realize a downlink transmission rate up to 1 Gbps or above.
In the LTE-A system, in order to simultaneously achieve backward compatibility with the LTE system and ultra-high-speed communications several times faster than transmission rates in the LTE system, the LTE-A system band is divided into “component carriers” of 20 MHz or below, which is the bandwidth supported by the LTE system. In other words, the “component carrier” is defined herein as a band having a maximum width of 20 MHz and as the basic unit of communication band. Moreover, in FDD (frequency division duplex) systems, “component carrier” in downlink (hereinafter, referred to as “downlink component carrier”) is defined as a band obtained by dividing a band according to downlink frequency band information in a BCH broadcasted from a base station or as a band defined by a distribution width when a downlink control channel (PDCCH) is distributed in the frequency domain. In addition, “component carrier” in uplink (hereinafter, referred to as “uplink component carrier”) may be defined as a band obtained by dividing a band according to uplink frequency band information in a BCH broadcasted from a base station or as the basic unit of a communication band of 20 MHz or below including a Physical Uplink Shared CHannel (PUSCH) in the vicinity of the center of the band and PUCCHs for LTE on both ends of the band. Note that the term “component carrier” may be also referred to as “cell” in English in 3GPP LTE-Advanced. In addition, “component carrier” may be also abbreviated as CC(s).
The LTE-A system supports communication using a band obtained by bundling several component carriers, so-called carrier aggregation (CA). Note that while a UL-DL configuration can be set for each component carrier, an LTE-A system compliant terminal (hereinafter, referred to as “LTE-A terminal”) is designed on the assumption that the same UL-DL configuration is set among a plurality of component carriers.
FIGS. 3A and 3B are diagrams provided for describing asymmetric carrier aggregation and a control sequence applied to individual terminals.
As illustrated in FIG. 3B, a configuration in which carrier aggregation is performed using two downlink component carriers and one uplink component carrier on the left is set for terminal 1, while a configuration in which the two downlink component carriers identical with those used by terminal 1 are used but uplink component carrier on the right is used for uplink communications is set for terminal 2.
Referring to terminal 1, a base station (that is, LTE-A system compliant base station (hereinafter, referred to as “LTE-A base station”)) and an LTE-A terminal included in the LTE-A system transmit and receive signals to and from each other in accordance with the sequence diagram illustrated in FIG. 3A. As illustrated in FIG. 3A, (1) terminal 1 is synchronized with the downlink component carrier on the left when starting communications with the base station and reads information on the uplink component carrier paired with the downlink component carrier on the left from a broadcast signal called system information block type 2 (SIB2). (2) Using this uplink component carrier, terminal 1 starts communications with the base station by transmitting, for example, a connection request to the base station. (3) Upon determining that a plurality of downlink component carriers need to be assigned to the terminal, the base station instructs the terminal to add a downlink component carrier. However, in this case, the number of uplink component carriers is not increased, and terminal 1, which is an individual terminal, starts asymmetric carrier aggregation.
In addition, in the LTE-A system to which carrier aggregation is applied, a terminal may receive a plurality of downlink data items on a plurality of downlink component carriers at a time. In LTE-A, studies have been carried out on channel selection (also referred to as “multiplexing”), bundling and a discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) format (may also be referred to as “PUCCH Format 3”) as a method of transmitting a plurality of response signals for the plurality of downlink data items. In channel selection, a terminal causes not only symbol points used for response signals, but also the resources to which the response signals are mapped to vary in accordance with the pattern for results of the error detection on the plurality of downlink data items. Compared with channel selection, in bundling, a terminal bundles ACK or NACK signals generated according to the results of error detection on the plurality of downlink data items (i.e., bundles by calculating a logical AND of the results of error detection on the plurality of downlink data items, provided that ACK=1 and NACK=0), and transmits response signals using one predetermine resource (see NPLs 6 and 7). In transmission using the DFT-S-OFDM format (PUCCH Format 3), a terminal jointly encodes (i.e., joint coding) the response signals for the plurality of downlink data items and transmits the coded data using the format (see, NPL 5). For example, a terminal may feed back the response signals (i.e., ACK/NACK) using channel selection, bundling or DFT-S-OFDM according to the number of bits for a pattern for results of error detection. Alternatively, a base station may previously configure the method of transmitting the response signals.
Furthermore, as shown in FIG. 4, the terminal transmits response signals using one of a plurality of component carriers. A component carrier that transmits such response signals is called “primary component carrier (PCC)” or “primary cell (PCell).” A component carrier other than the primary component carrier is called “secondary component carrier (SCC)” or “secondary cell (SCell).” For example, the PCC (PCell) is a component carrier that transmits broadcast information on a component carrier that transmits response signals (e.g., system information block type 1 (SIB1)).
The following two methods are considered as a possible method of transmitting response signals in uplink when a terminal receives downlink assignment control information via a PDCCH and receives downlink data.
One is a method to transmit response signals using a PUCCH resource associated in a one-to-one correspondence with a beginning CCE index nCCE (or nCCE+1 adjacent thereto) of a control channel element (CCE) occupied by the PDCCH (i.e., implicit signaling) (hereinafter, method 1). More specifically, when DCI intended for a terminal served by a base station is allocated in a PDCCH region, each PDCCH occupies a resource consisting of one or a plurality of contiguous CCEs. In addition, as the number of CCEs occupied by a PDCCH (i.e., the number of aggregated CCEs: CCE aggregation level), one of aggregation levels 1, 2, 4 and 8 is selected according to the number of information bits of the assignment control information or a propagation path condition of the terminal, for example.
The other method is to previously report a PUCCH resource to each terminal from a base station (i.e., explicit signaling) (hereinafter, method 2). To put it differently, each terminal transmits response signals using the PUCCH resource previously indicated by the base station in method 2.
In method 2, PUCCH resources common to a plurality of terminals (e.g., four PUCCH resources) may be previously indicated to the terminals from a base station. For example, terminals may employ a method to select one PUCCH resource to be actually used, on the basis of a transmit power control (TPC) command of two bits included in DCI in SCell. In this case, the TPC command is called an ACK/NACK resource indicator (ARI). Such a TPC command allows a certain terminal to use an explicitly signaled PUCCH resource in a certain frame while allowing another terminal to use the same explicitly signaled PUCCH resource in another subframe in the case of explicit signaling.
Regarding PUCCH Format 3 and channel selection, which are methods of indicating results of error detection when carrier aggregation is applied, a method of determining PUCCH resources will be described with reference to FIGS. 5A and 5B and FIGS. 6A and 6B.
In PUCCH Format 3, as shown in FIG. 5B, a terminal indicates results of error detection corresponding to a plurality of downlink data items for each downlink component carrier received in a maximum of five downlink component carriers to a base station using PUCCH Format 3 resources or PUCCH Format 1b resources. To be more specific, the base station indicates a TPC command of PUCCH in a field including a 2-bit TPC command (also referred to as “TPC field”) with a PDCCH that specifies the PDSCH of PCell. That is, this field is not used as an ARI. The base station indicates a PUCCH resource (PUCCH Format 1b resource) associated in a one-to-one correspondence with the beginning CCE index nCCE of the CCE occupied by the PDCCH. Moreover, the base station previously sets four PUCCH resources (PUCCH Format 3 resources) for the terminal and indicates an ARI with 2 bits of the TPC field in the PDCCH that specifies the PDSCH of SCell. That is, this field is not used as a TPC command of PUCCH. Note that in FIG. 5B, the ARI included in the PDCCH that specifies the PDSCH of SCell is referred to as ARI1 for convenience. The terminal determines which resource of the previously set four PUCCH resources (PUCCH Format 3 resources) should be used for PUCCH transmission according to an ARI indicated by the PDCCH. Note that the base station indicates the same value to the terminal as the value of the ARI included in the PDCCH specifying the PDSCHs of a plurality of SCells. This allows the terminal to always determine a single PUCCH Format 3 resource.
In PUCCH Format 3, when detecting a PDCCH specifying the PDSCH of at least one SCell, the terminal indicates results of error detection (in the FDD system, a maximum of 10 bits (=5 CCs×2 CWs)) to the base station using the above-described PUCCH Format 3 resources. On the other hand, when detecting only a PDCCH specifying the PDSCH of PCell, the terminal indicates results of error detection (a maximum of 2 bits (=1 CC×2 CWs)) to the base station using a PUCCH Format 1b resource associated in a one-to-one correspondence with the beginning CCE index nCCE of the PDCCH.
The PUCCH Format 1b resource is a PUCCH resource optimized for transmission of results of error detection of up to a maximum of 2 bits and can also be orthogonalized with a maximum of 48 resources. While the PUCCH Format 3 resource is a PUCCH resource optimized for transmission of more results of error detection, it can orthogonalize only up to a maximum of 4 resources. When the number of bits of results of error detection is small, using PUCCH resources optimized for a smaller number of bits of results of error detection allows the PUCCH resources to be orthogonalized to more resources and thereby increases the utilization efficiency of PUCCH resources. Moreover, required PUCCH transmission power at the terminal necessary to satisfy required quality in the base station can also be reduced.
Furthermore, when detecting only a PDCCH that specifies a PDSCH of PCell, the terminal indicates results of error detection to the base station using a PUCCH Format 1b resource associated in a one-to-one correspondence with the beginning CCE index nCCE of the PDCCH, whereby, results of error detection for at least PDSCH of PCell can be indicated without inconsistency between the base station and the terminal even for a period during which the understanding of the setting of the number of CCs differs between the base station and the terminal (hereinafter may be expressed as “supporting LTE fallback”).
To be more specific, there is a period during which the understanding of the number of CCs configured in the terminal differs between the base station and the terminal (uncertainty period or misalignment period). The base station indicates, to the terminal, a message indicating a reconfiguration so as to change the number of CCs (e.g., from 1 CC to 2 CCs or vice versa) and, upon reception of the message, the terminal understands that the number of CCs has been changed and indicates, to the base station, a message indicating completion of the reconfiguration of the number of CCs. The period in which the understanding about the number of CCs configured for a terminal is different between a base station and the terminal stems from the fact that the base station understands, upon reception of the message, for the first time, that the number of CCs configured for the terminal has been changed. In a case where the terminal detects only a PDCCH specifying the PDSCH of PCell in common before and after the change of the number of CCs (e.g., FIG. 5A and FIG. 5B or FIG. 6A and FIG. 6B), if the terminal operates so as to use a PUCCH Format 1b resource associated in a one-to-one correspondence with the beginning CCE index nCCE of the PDCCH, the terminal can indicate results of error detection for at least PDSCH of PCell to the base station without inconsistency (that is, supporting LTE fallback) even for a period during which the understanding of the number of CCs differs.
In channel selection, as shown in FIG. 6B, the terminal indicates results of error detection corresponding to a plurality of downlink data items for each downlink component carrier received in a maximum of two downlink component carriers to the base station using four PUCCH Format 1b resources. In channel selection, not only symbol points used for response signals but also PUCCH Format 1b resources to which response signals are mapped are changed in accordance with a pattern (combination of ACK/NACK) of the results of error detection regarding a plurality of downlink data items. In channel selection, the base station indicates a TPC command of PUCCH in a field including a 2-bit TPC command (also referred to as “TPC field”) in a PDCCH of PCell. That is, this field is not used as an ARI. The base station specifies the PUCCH resources (PUCCH Format 1b resources) associated in a one-to-one correspondence with the beginning CCE index nCCE of the CCE occupied by the PDCCH and CCE index nCCE+1 adjacent thereto respectively (PUCCH resources 0 and 1 in FIGS. 6A and 6B). The base station previously sets four PUCCH resource pairs (PUCCH Format 1b resource pairs) for the terminal and indicates an ARI using 2 bits of the TPC field in a PDCCH of SCell. That is, this field is not used as a TPC command of PUCCH. In FIGS. 6A and 6B, an ARI included in a PDCCH of SCell is referred to as ARI1 for convenience. The terminal determines one resource pair of the four previously set PUCCH resource pairs (PUCCH Format 1b resource pairs) according to the ARI indicated by the PDCCH (PUCCH resources 2 and 3 in FIG. 6B).
In recent years, it has become common to transmit not only audio data but also large-volume data, such as still image data and moving image data in cellular mobile communication systems in response to spread of multimedia information. In LTE-Advanced (Long Term Evolution Advanced), studies are being actively conducted on achieving high transmission rates using wide radio bands, multiple-input multiple-output (MIMO) transmission technique and interference control technique.
In consideration of the fact that various devices for M2M (machine to machine) communication or the like are introduced as radio communication terminals and the number of terminals multiplexed by a MIMO transmission technique, there is concern about a shortage of resources in a region to which PDCCH (Physical Downlink Control CHannel: downlink control channel) used for control signals are mapped (that is, “PDCCH region”). When control signals (PDCCHs) cannot be mapped due to this shortage of resources, data cannot be assigned to terminals. For this reason, even when there are resource regions available for data mapping, they cannot be used, and the system throughput may decrease.
As a method of solving this shortage of resources, studies are being carried out on a possibility of arranging control signals intended for terminals served by the base station in PDSCH regions as well. Resource regions in which control signals intended for terminals served by the base station are mapped (resource regions available to both a control channel and a data channel) are called “enhanced PDCCH (ePDCCH) regions.” Thus, by mapping control signals in the data region (that is, ePDCCH), it is possible to achieve transmission power control over control signals transmitted to terminals located in the vicinity of a cell edge or control over interference provided with transmitted control signals to other cells or control over interference provided from the other cells to the own cell.
In LTE, DL assignment indicating downlink data assignment and UL grant indicating uplink data assignment are transmitted by PDCCHs.
In LTE-Advanced, DL assignment and UL grant are assigned to an ePDCCH as well as PDCCH. Studies are being carried out on a possibility of dividing resources to which DL assignment is mapped and resources to which UL grant is mapped in an ePDCCH in the frequency domain.
Studies are being carried out on a possibility of “localized assignment” whereby ePDCCHs are collectively assigned at positions close to each other in a frequency band and “distributed assignment” whereby ePDCCHs are assigned in a frequency band in a distributed manner, as ePDCCH assignment methods (e.g., see FIG. 7).